1. Field of the Art
The present invention relates to a coil arrangement for correction of magnetic field used in an imaging equipment utilizing nuclear magnetic resonance (NMR) or the like, and more particularly to a coil arrangement for correction of magnetic field wherein when the direction of a fundamental magnetic field is assumed as Z axis of the Cartesian or rectangular coordinate system, the coil arrangement is provided with at least one pair of ring-shaped coils for correcting error magnetic field components with respect to Z axis included in the fundamental magnetic field.
2. Prior Art
In imaging equipment using NMR, a high uniform or homogeneous magnetic field is required. For this reason, the coil arrangement for producing a fundamental magnetic field is designed so as to produce a high homogeneous magnetic field. However, when the coil arrangement is actually excited, the magnetic field is disturbed due to the influence of magnetic material such as iron existing therearound to produce error magnetic field components. To cancel such error magnetic components to improve homogeneity of the magnetic field, a coil arrangement for correction of magnetic field is used.
The coil arrangement of this kind is described, for example, in the paper entitled "SUPERCONDUCTING MAGNETS FOR NMR IMAGING AND IN-VIVO SPECTROSCOPY" reported by the Oxford Instruments Group in "Proceedings of the Tenth Internationl Cryogenic Engineering Conference (HELSINKI, 1984)".
In this paper, in the case of correcting respective error magnetic components H.sub.0, H.sub.1, H.sub.2, . . . from the zeroth order with respect to Z axis, magnetic field correction coils dedicated for respective components are used for making corrections per each component. For this reason, for example, for coils for correction of the first to fourth order magnetic fields, one pair, four pairs, two pairs and four pairs, are required respectively. When an attempt is made to correct a magnetic field using a coil producing only a certain specified order component in accordance with the prior art, other order components would be produced as well, although they are small. Thus, there occurs the inconvenience that such other components degrade the homogeneity of the magnetic field. A further problem is that since correction is made per each order component as previously described, a large number of coils are required in correspondence with the order components to be corrected in order to reduce various order components to only a specified single component.